1. Field of the Invention
This invention relates to a manufacturing method of a MIM (Metal-Insulator-Metal) nonlinear device, and to an MIM nonlinear device and a liquid crystal display device.
2. Related Art
Generally, active matrix liquid crystal display devices comprise two substrates, between which liquid crystal is filled. On one substrate, a switching device is provided for each pixel region to form a matrix array, and on the other substrate, a color filter is formed. The orientation of the liquid crystal is controlled in each pixel region, thereby displaying prescribed information. As the switching device, a three-terminal device, such as a TFT (Thin Film Transistor), or a two-terminal device, such as MIM nonlinear device, are typically used. A MIM nonlinear device is advantageous in responding to the demand for a large-sized screen and reduced manufacturing cost. Moreover, a MIM nonlinear device has another advantage of eliminating crossover short-circuit between the scan line and the data line because scan lines and data lines are separately provided on different substrates. That is, scan lines are provided on the substrate having a matrix array formed thereon while data lines are provided on the other substrate.
FIG. 14 shows an example of the conventional active matrix liquid crystal display device 100 using an MIM nonlinear device, in which a matrix is formed by a plurality of scan lines 74 connected to the scan line driving circuit 72, and a plurality of data lines 78 connected to the data line driving circuit 76. Pixel region 80 is formed in each element of the matrix. Pixel region 80 includes a MIM nonlinear device 50 connected to the data line 78 at one end, and a liquid crystal display element 60 connected between the MIM nonlinear device 50 and the scan line 74. Liquid crystal display element 60 is driven based on the differential voltage between the signal applied to the scan line 74 and the signal applied to the data line 78. If the threshold voltage of liquid crystal element 60 is represented as (Vb), and the threshold voltage of MIM nonlinear device 50 is represented as (Vth), and if the voltage at both terminals of the liquid crystal display element 60, which turns on the liquid crystal display element 60, is represented as (Vb+.DELTA.V), then the liquid crystal display element 60 is in the ON state when the differential voltage is (Vb+Vth+.DELTA.V) during a selected period, while the liquid crystal display element 60 is in the OFF state when the differential voltage is (Vb+Vth). During a non-selected period, the differential voltage is set to less than (Vth) to maintain the state decided during the selected period.
FIG. 15 is a cross-sectional view of the active matrix liquid crystal display device 100 using an MIM nonlinear device. Liquid crystal layer 40 is positioned between the electrode substrates 10 and 30. Electrode substrate 10 comprises a transparent board 12, MIM nonlinear devices 50 formed on the transparent board 12, and pixel electrodes 22 connected to the corresponding MIM nonlinear device 50. MIM nonlinear device 50 is composed of a Ta electrode layer 16 formed on the transparent electrode 12, a Ta.sub.2 O.sub.5 film 18 formed on the Ta electrode 16, and a Cr electrode layer 20 formed on the Ta.sub.2 O.sub.5 film 18. Ta.sub.2 O.sub.5 film 18 is formed on the surface of the Ta electrode layer 16 through anodic oxidation of the Ta electrode layer 16 so that the film thickness becomes uniform without generating pin holes. (See Japanese Patent Application Laid-Opens 5-297389 and 5-313207.)
With the conventional method, MIM nonlinear device 50 having such a structure is manufactured as follows: forming a tantalum oxide layer 14 with a thickness of about 1000 .ANG. by depositing a tantalum layer on the transparent substrate 12 by sputtering, followed by heat oxidation; depositing a tantalum layer up to about 3000 .ANG. by sputtering, and patterning the tantalum layer to form a Ta electrode layer 16; performing anodic oxidation to the Ta electrode layer 16 to form a Ta.sub.2 O.sub.5 anodic oxidation film 18; and depositing chromium film with a thickness 1500 .ANG. by sputtering and patterning the chromium film to form a Cr electrode layer 20, thereby completing a MIM nonlinear device 50.
IEEE Trans Electron Device, Vol. ED28, pp. 736-739, June 1981, proposes a technique for doping nitrogen into Ta electrode layer 16 composing an MIM nonlinear device, in order to improve the nonlinear characteristics of the MIM nonlinear device. However, this technique requires highly advanced technology of tantalum sputtering, and makes it difficult to manufacture the MIM nonlinear device with good repeatability.
Japanese Patent Application Laid-Open 63-50081 proposes a technique of anodic oxidation of the tantalum film, which is followed by heat treatment at a temperature from 400.degree. C. to 600.degree. C. in the nitrogen atmosphere, for improving the nonlinear characteristic of the MIM nonlinear device. However, simply conducting heat treatment at 400.degree.-600.degree. C. in the nitrogen atmosphere after the anodic oxidation of the tantalum film can not achieve an adequate nonlinear characteristic and a satisfactory resistance characteristic in the OFF state, which are required for obtaining an excellent image quality. Thus, further improvement of the nonlinear characteristic and resistance characteristic has been desired.
Therefore, the objective of the invention is to provide a method for manufacturing an MIM nonlinear device, which is capable of improving the nonlinear characteristic and resistance characteristic in the OFF state of a MIM nonlinear device, and to provide a liquid crystal display device using an MIM nonlinear device with improved a nonlinear characteristic and resistance characteristic in the OFF state.